Sintered ferrite magnets are used in various applications such as motors, power generators, speakers, etc. Known as typical sintered ferrite magnets are Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) each having a hexagonal M-type magnetoplumbite structure. These sintered ferrite magnets are produced from starting materials comprising iron oxide, carbonates of strontium (Sr) or barium (Ba), etc. by a powder metallurgy method at a relatively low cost.
Recently, to provide smaller, lighter-weight, higher-performance electronic parts for automobiles, electric appliances, etc. for environmental advantages, sintered ferrite magnets are required to have higher performance. Particularly desired for motors for automobiles are sintered ferrite magnets having, in addition to high residual magnetic flux density Br, such high coercivity HcJ as to avoid demagnetization when thinned.
JP 10-149910 A and JP 11-154604 A propose sintered Sr ferrite magnets having improved HcJ and Br, in which part of Sr is substituted by a rare earth element such as La, etc., and part of Fe is substituted by Co.
Because the Sr ferrite described in JP 10-149910 A and JP 11-154604 A comprising Sr partially substituted by a rare earth element such as La, etc., and Fe partially substituted by Co, etc., which is hereinafter referred to as “Sr—La—Co ferrite,” has excellent magnetic properties, it has been getting used for various applications in place of conventional Sr ferrite and Ba ferrite, but further improvement in magnetic properties is desired.
In addition to the above Sr ferrite and Ba ferrite, Ca ferrite is also known for sintered ferrite magnets. It is known that Ca ferrite having a composition represented by the formula of CaO—Fe2O3 or CaO-2Fe2O3 has a stable structure, constituting hexagonal ferrite by the addition of La. However, it has magnetic properties on substantially the same levels as those of conventional Ba ferrite, not sufficiently high.
To improve the Br and HcJ of Ca ferrite as well as the temperature characteristics of HcJ, Japanese Patent 3181559 discloses Ca ferrite with part of Ca substituted by a rare earth element such as La, etc. and part of Fe substituted by Co, etc. (hereinafter referred to as “Ca—La—Co ferrite”), which has an anisotropic magnetic field HA of 20 kOe or more, higher by 10% or more than that of Sr ferrite.
However, the Ca—La—Co ferrite has substantially the same Br and HcJ as those of Sr—La—Co ferrite, despite a higher anisotropic magnetic field HA than that of the Sr—La—Co ferrite. In addition, having an extremely poor squareness ratio, it does not meet high coercivity and a high squareness ratio simultaneously, failing to be used for various applications such as motors, etc.
To improve the magnetic properties of the Ca—La—Co ferrite, various proposals have been made. For example, JP 2006-104050 A proposes Ca—La—Co ferrite having optimized composition ratio and molar ratio (n) of each element with a particular ratio of La to Co, WO 2007/060757 proposes Ca—La—Co ferrite with part of Ca substituted by La and Ba, and WO 2007/077811 proposes Ca—La—Co ferrite with part of Ca substituted by La and Sr.
Although any Ca—La—Co ferrites of JP 2006-104050 A, WO 2007/060757 and WO 2007/077811 have higher magnetic properties than those of the Ca—La—Co ferrite proposed by Japanese Patent 3181559, demand for higher performance is increasingly stronger recently, requiring further improvement in magnetic properties.
In sintered ferrite magnets, the addition of SiO2, CaCO3, etc. as sintering aids is known to change balance between Br and HcJ, which are in a trade-off relation. To obtain high Br, it is effective to reduce the amounts of sintering aids acting as non-magnetic components in a range having liquid phase components necessary for sintering, or to increase the amount of CaCO3 relative to that of SiO2. However, such measures make it difficult to maintain fine sintered structures, resulting in low HcJ. On the other hand, to obtain high HcJ, it is effective to increase the amounts of sintering aids, or to increase the amount of SiO2 relative to that of CaCO3. However, such measures increase the amounts of non-magnetic components or lower sinterability, inevitably resulting in decrease in Br and a squareness ratio Hk/HcJ, wherein Hk is the value of H at J of 0.95Br on a curve of J (intensity of magnetization) to H (intensity of a magnetic field) in the second quadrant.
In conventional sintered ferrite magnets, particularly in Sr—La—Co ferrite and Ca—La—Co ferrite proposed recently, the amounts of sintering aids such as SiO2, CaCO3, etc. are generally as small as possible to maintain high Br. For example, JP 2006-104050 A describes that 0.3-1.5% by mass (calculated as CaO) of CaCO3 and 0.2-1.0% by mass of SiO2 are added preferably at the time of pulverizing a calcined body, and WO 2007/060757 describes that 0.2-1.5% by mass (0.112-0.84% by mass when calculated as CaO) of CaCO3, and 0.1-1.5% by mass of SiO2 are added preferably at the time of pulverizing a calcined body.
However, JP 2006-104050 A and WO 2007/060757 describe in Examples only Ca—La—Co ferrite magnets, in which the amounts of SiO2 and CaCO3 (calculated as CaO) are respectively 0.9% or less by mass. Because emphasis is placed on the improvement of the Br of these ferrite magnets, the addition of SiO2 and CaCO3 in amounts of more than 0.9% by mass is not contemplated, providing no information about the magnetic properties (Br, HcJ and Hk/HcJ) of such Ca—La—Co ferrite magnets.